Hybrid powertrains include an engine, one or more motors useful for propulsion and an energy storage device to power the motor or motors. One type of hybrid powertrain is a hybrid electric powertrain, which includes an electric motor/generator and a battery as the energy storage device. Another type of hybrid powertrain is a fluid hybrid powertrain, such as a hydraulic hybrid, using one or more fluid motors and a fluid accumulator as the energy storage device.
A specific type of hybrid electric powertrain uses an electrically variable transmission incorporating one or more differential gear sets. Power from the engine and/or from the motor/generators flows through different members of the differential gear set. A controller may control torque-transmitting mechanisms in various engagement schemes to offer a combination of operating modes, including electrically-variable and fixed ratio modes.
Vehicle accessories, such as a transmission pump, an air conditioning compressor and a power steering pump, are often driven by the engine crankshaft in a conventional vehicle. On a hybrid vehicle, because the engine is intentionally stopped under some driving conditions, an alternate way of powering such vehicle accessories is necessary. For example, these accessories are sometimes provided with individual electric motors or with a common electric motor to run several accessories.